The invention relates to a carbonylation process of pentenenitrile to prepare 5-cyanovaleric acid or its ester in the presence of a catalyst system and to a process to prepare xcex5-caprolactam wherein such a carbonylation process is used.
Commercial processes for the preparation of xcex5-caprolactam use either phenol or cyclohexane as starting compounds. A disadvantage of these routes is that ammonium sulphate is produced as an unwanted by-product. Furthermore, these known processes include numerous process steps, which makes the preparation of xcex5-caprolactam a laborious and costly process. Thus, in the field of preparing xcex5-caprolactam there is a great need for a new route based on butadiene.
In recent patent literature, a butadiene based preparation of xcex5-caprolactam is described wherein first a pentenoate ester is prepared by carbonylation of butadiene, which in turn is reacted to 5-formylvalerate ester in a hydroformylation step. The 5-formylvalerate ester is subsequently converted to 6-aminocaproic acid or its ester in a reductive amination step. 6-aminocaproic acid or its ester is subsequently reacted to xcex5-caprolactam upon heating in an aqueous medium. According to U.S. Pat. No. 5,693,851, which describes a palladium catalysed carbonylation of butadiene at 140xc2x0 C., the best selectivity to methyl 3- and 4-pentenoate ester is about 93%. According to U.S. Pat. No. 6,018,081, which describes a rhodium catalysed hydroformylation of methyl pentenoate ester, the best selectivity to methyl 5-formylvalerate ester is 81%. According to EP-A-729943 and WO-A-9837063, a 100% conversion of methyl 5-formylvalerate to xcex5-caprolactam is achievable in the reductive amination and cyclisation steps. Based on butadiene the overall selectivity is thus at most about 75%. This means that 25% of the starting butadiene is converted to by-products. It will be clear that this overall selectivity will have to be significantly improved for a commercial application.
DE-A-19840253 describes the possibility of a process to prepare caprolactam starting from 5-cyanovaleric acid and its esters. Through hydrogenation to 6-aminocaproic acid, respectively 6-aminocaproic ester and elimination of the water or alcohol caprolactam can be obtained.
DE-A-19840253 further relates to a process to prepare cyanovaleric acids or esters by reacting pentenenitril with water or an alkanol and carbon monoxide in the presence of a catalyst system comprising a palladium (II) compound, a bidentate diphosphine and a source of anions. On page 3, lines 29-36, DE-A-19840253 mentions an extensive list of possible bidentate diphosphine ligands, including for example 1,2-bis(di-n-butylphosphino)ethane, 1,3-bis(dimethylphosphino)propane, 1,3-bis(di-isopropylphosphino)propane and 1,2-bis(dicyclohexylphosphino)ethane, as well as 1,3-bis(di-tert-butylphosphino)propane. In addition DE-A-19840253 mentions, on page 4, lines 55 to 62, a whole range of possible sources of anions, of which weak organic acids with a pKa of 3.5 or more, such as 9-anthracenecarboxylic acid, are preferred.
In its examples DE-A-19840253 describes the preparation of methyl 5-cyanovalerate by reacting 3-pentenenitril with methanol and carbon monoxide in the presence of Palladium(II)acetate, 9-anthracenecarboxylic acid, and 1,2-bis(dicyclohexylphosphino)ethane or a mixture of 1,2-bis(1,5-cyclooctylenephosphino)ethane and 1,2-bis(1,4-cyclooctylenephosphino)ethane at a temperature of 150xc2x0 C. At conversions lying in the range from 40 to 90%, selectivities to the desired methyl 5-cyanovalerate in the range from 70 to 72% were obtained.
U.S. Pat. No. 4,950,778 describes a process to prepare 5-cyanovaleric acid by reacting 3-pentenenitrile with water and carbon monoxide in the presence of a cobalt catalyst at a pressure of 136 bar and a temperature of 200xc2x0 C. At a conversion of 87.4%, the selectivity to the undesired branched C6 acids was 9.1% and to the undesired valeronitril was 9.6%.
U.S. Pat. No. 5,434,290 describes a process to prepare methyl 5-cyanovalerate by reacting 3-pentenenitrile with methanol and carbon monoxide in the presence of a cobalt catalyst at a pressure of 200 bar and a temperature of 160xc2x0 C. At a conversion of 66%, the selectivity to the desired methyl 5-cyanovalerate was about 89%.
Some of the disadvantage of the above processes are the high operating pressure and/or temperature, the use of high concentrations of cobalt carbonyl compounds and/or the low selectivity at a relatively low conversion.
U.S. Pat. No. 5,679,831 describes the carbonylation of methyl 3-pentenoate to dimethyl adipate by reacting the methyl-3-pentenoate with methanol and carbon monoxide in the presence of a catalyst system consisting of palladium, 1,1xe2x80x2-bis(diisopropylphosphino)ferrocene and p-toluene sulphonic acid at a pressure of 60 bar and a temperature of 130xc2x0 C. At 99% conversion, a 83% selectivity to dimethyl adipate was obtained. Another experiment performed at 90xc2x0 C. illustrated a selectivity of 84% to adipate at a 71% conversion of pentenoate. All experiments were performed starting with pentenoate and with an acid to palladium molar ratio of above 10. Pentenenitrile is mentioned as a possible substrate instead of pentenoate. However if pentenenitrile is used instead of methyl-3-pentenoate using the same ligand and under the conditions of the examples, no catalyst activity is observed. Another disadvantage is that because of the high acid concentration the reaction mixture is corrosive and more ligand degradation results due to quartanization of the phosphine compound with the acid and the olefinic compound.
EP-A-495548 describes the carbonylation of propene by reacting propene with methanol and carbon monoxide in the presence of palladium, 1,3-bis(di-tert.butylphosphino)propane and methylsulphonic acid at a pressure of 30 bar and a temperature of 60xc2x0 C. The selectivity to the desired linear methylbutanoate was 86%.
A process for preparing a 5-cyanovaleric acid or its ester is provided, comprising reacting pentenenitrile with carbon monoxide and water and/or an alcohol in the presence of a catalyst system, comprising
(a) a source of Group VIII metal and
(b) a bidentate phosphine, arsine and/or stibine ligand, wherein the bidentate ligand has the general formula (I):
R1R2xe2x80x94M1xe2x80x94Rxe2x80x94M2xe2x80x94R3R4xe2x80x83xe2x80x83(I)
xe2x80x83wherein M1 and M2 are independently P, As or Sb, R is a divalent organic bridging group, which bridging group comprises a chain of 3 to 5 atoms directly connecting the 2 phosphorus atoms, which chain consists of carbon atoms and optionally a nitrogen, oxygen or sulphur atom or a silano or dialkylsilicon group, which alkyl groups independently comprise from 1 to 4 carbon atoms, and R1-R4 represent the same or different optionally substituted tertiary alkyl groups,
(c) an acid having a pKa less than 3, as measured at 18xc2x0 C. in an aqueous solution.
Also provided is a process to prepare xcex5-caprolactam comprising reducing 5-cyanovaleric acid or ester obtained above to 6-aminocaproic acid or ester, and then cyclisation of the 6-aminocaproic acid or ester to xcex5-caprolactam.